1. Field of the Invention
The present invention relates to a battery using, as an anode active material, any of metal elements or metalloid elements each of which can form an alloy with a light metal such as lithium (Li), alloys of these elements, and compounds of these elements.
2. Description of the Related Art
In recent years, as electron technologies advance, increase in performance, miniaturization, and portability of electronic devices are being vigorously improved. In association with the improvements, researches of a rechargeable secondary battery as a power source which can be used conveniently and economically for long time are being conducted. Hitherto, secondary batteries such as a lead battery, an alkaline battery, a lithium ion secondary battery, and the like are widely known. Among them, attention is paid to a lithium ion secondary battery as a battery capable of realizing high output and high energy density.
In a lithium ion secondary battery, conventionally, a carbonaceous material such as non-graphitizable carbon or graphite is widely used as an anode active material. Although the carbonaceous materials exhibit relatively high capacity and excellent cycle characteristic, in association with increase in capacity in recent years, a higher capacity is demanded, and research is being conducted. For example, Japanese Unexamined Patent Application No. 8-315825 discloses a technique of increasing the capacity by selecting a carbonaceous material and manufacturing conditions. According to the technique, however, the discharge potential of an anode with respect to lithium is 0.8 V to 1.0 V. Since the battery discharge voltage when a battery is constructed becomes lower, it is difficult to largely improve the energy density. Further, a charge/discharge curve shows a large hysteresis and there is a problem such that the energy efficiency in each charge/discharge cycle is low.
On the other hand, as other anode active materials, lithium alloys applied to be reversibly generated/decomposed electrochemically have been widely studied. The lithium alloys include Li—Al alloys, and Si alloys disclosed in U.S. Pat. No. 4,950,566.
The Li—Al alloy and Li—Si alloy expand/contract in association with charge/discharge much more than the carbonaceous materials. Consequently, as the charge/discharge cycle repeats, the anode is pulverized, and it causes a problem such that the cycle characteristic is extremely low. To improve the cycle characteristic, therefore, methods of adding an element which is not involved in expansion/contraction with doping/undoping of lithium into/from lithium alloy have been examined. For example, Japanese Unexamined Patent Application No. 6-325765 proposes LixSiOy (x≧0, 2>y>0), Japanese Unexamined Patent Application No. 7-230800 proposes LixSi1−yM1yOz (x≧0, 1>y>0, 0<z<2, where M1 denotes a metal element such as Ti, W, or Mn), and Japanese Unexamined Patent Application No. 7-288130 proposes an Li—Ag—Te alloy. However, even if the methods are employed, it is difficult to suppress deterioration in cycle performance due to expansion/contraction which occurs in association with charge/discharge and, in actual circumstances, the advantage of high capacity of a lithium alloy is not fully utilized.
Japanese Unexamined Patent Application No. 11-120705 reports a high-capacity anode using a 4B group compound except for carbon, containing one or more nonmetal elements. The problem of deterioration in the charge/discharge cycle performance is not solved also by the reported technique.